In designing the drive train for a passenger car with a manual transmission, in recent years greater attention has been paid to the problem of damping transmission noise such as rapping at idle and rattle. Rapping at idle occurs when the car is stationary and the engine is idling, while rattle occurs when the car is driven and the drive train is loaded. Both are caused by the fact that the harmonic torque of the engine gives rise to a torsional oscillation cycle in the gearbox when the primary shaft of the gearbox is subjected to an angular acceleration by the primary shaft gear and its input shaft is then retarded by the engine and by the drag torque of the gearbox.
A known method of dampening rapping at idle and rattle is to reduce these torsional oscillations by dividing the engine flywheel into two masses with an intermediate torque transmitting spring and damping system. Such a system replaces the spring and damping device arranged in the disc center of a conventional clutch. The double mass flywheel with an intermediate spring system functions as a mechanical "low pass filter" and by suitable dimensioning of the components it can reduce the natural frequency of the torsional oscillations from 40-70 Hz, corresponding to the rpm range at which the engine operates under normal conditions, to about 15 Hz, which lies below the normal idle rpm of the engine.
In a known double mass system functioning according to the principle described, the spring system itself has two different types of springs, lying in series with each other and having different spring characteristics. When idling under absence of torque, torque is transmitted from the weaker to the stronger springs, producing a natural resonance in the system which is significantly below the idle rpm of the engine. When the engine is driving or during engine braking, the weaker springs are completely compressed and the damping during torque transmission is effected entirely by the stronger springs. The natural resonance of the system will then be above the idle frequency but below the frequency of the lowest rpm for the loaded engine. A disadvantage of the known system is, however, poor drivability, i.a. because of the fact that the soft springs make the relative rotation between the flywheel masses be so great during certain operating conditions that the drive train will be "stretched" and the driving will be "wobbly". The fewer the cylinders are and the higher the engine torque is in the system, the greater the problem will be.